The present invention relates to a process for charging a multicell battery by means of a battery recharging device and a device, controlled by a microcontroller, which contains a potential-free recharging device to analyze the charge state of such a battery having a plurality of cells or battery sections.
In the course of charging and discharging series-connected batteries, the terminal voltages of the individual cells or battery sections are different because of unavoidable different structural chemical and electrical parameters of the individual cells. For this reason, when using conventional chargers which, only utilize the terminal voltages of a large number of series-connected cells for controlling the charging process in accordance with known characteristic charging curves (UI-, W-, etc.), an unnoticed, increasingly varying charge of the cells occurs over time. This can go so far that the polarity of individual cells is changed. Hermetically sealed cells cause a particular problem in this case because they require the maintenance of a maximum charging voltage for preserving the electrolyte. This is further described in EP 0 339 446. In addition, when a multi-cell battery fails, it is difficult and time consuming to find the defective battery cell. Also, a determination of the reason for the failure is fundamentally impossible. Another problem occurs when individual cells are replaced by fresh ones having parameters which can widely vary in comparison to the remaining cells. This effect is particularly known to occur in Pb gel cells, the initial capacity of which is only about 70% of the final value, and which attain this final capacity only after 20 to 50 cycles. These cells cannot be satisfactorily charged by means of conventional chargers, so that the manufacturer is forced to make so-called pre-cycled cells or battery sections available.
The invention concerns two problems, namely the measuring, diagnosis or analysis of batteries, in particular the determination of the instantaneous capacity, and the directed recharging of selected cells or battery sections. A number of processes are known which infer the charge state of a composite battery from the terminal quality of the entire battery. These are described in EP 0 067 589, EP 0 071 816 and EP 0 090 699. Another arrangement is described in EP 0 112 242 which refers to the measurement of the capacity of a battery composed of series-connected individual elements. In this prior arrangement, the entire battery is loaded by means of a bleeder. Then all elements are cyclically addressed by means of a sequencer and the terminal voltages of all elements are sequentially supplied to a comparator. In addition, the discharging time is monitored. When the terminal voltage of an element falls below a threshold value, the cyclic measuring process is interrupted. The reference potential for the measurement of each cell voltage is the ground potential, so that with an increase in the number of elements the accuracy of this method is greatly reduced. Furthermore, a quantitative statement of the capacity is impossible and only the threshold value is monitored. Recharging of individual cells during the charging of the composite battery is not possible with this arrangement. In the circuit described in EP 0 277 321, the wiring outlay in connection with the measurement of a plurality of individual elements is reduced in that each cell or group of cells is connected with a measuring circuit which is triggered via a common control line and/or measuring line. These values are sequentially provided via a common measurement line to a common evaluation device. The battery charging system described in EP 0 074 444 shows a microprocessor and one or more sensors for detecting and further processing electrical signals in order to obtain output signals therefrom, which can be used for controlling the charging and for display. The computer controlled installation in accordance with EP 0 314 155 is used for charging a plurality of batteries, which may be of different types and are not electrically connected in series. The type and initial load of each battery is first determined and then the batteries are charged in the inverse order of their initial charge. The systems disclosed in EP 0 067 590, EP 0 121,325, EP 0 012 315, EP 0 181 112, EP 0 293 664 and EP 0 336 381 partially represent very expensive solutions, but are all based on the terminal voltage of the composite battery. A charging process is disclosed in EP 0 361 859 which permits charging and measuring of a plurality of batteries with the aid of a microprocessor. An automatically controlled microprocessor battery testing device BAT-CAT is available commercially from the firm ANPICO. This device is used exclusively for battery diagnoses. The cell voltages are read in via a plurality of analog inputs (measuring plugs) with high resolution (12 bit). Another battery charging and control system (BL+CS) is available commercially from Hildebrand Industrie Electronic in Dietlikon, Switzerland. With this device the individual cells or battery sections are connected via bipolar relays with a processor-controlled measuring and charging device, wherein one relay per cell is provided.
It is an object of the invention to provide a process for charging a multi-cell battery of the above mentioned type, by means of which overcharging of individual cells of the battery can be assuredly prevented, so that the battery as a whole is protected and its service life extended.